This invention relates to an improved process for making crosslinked branched polyesters, and their use as a chewing gum base and in non-food applications.
Aromatic polyesters such as poly(ethylene terephthalate) (PET) are high melting and which have a high degree of crystallinity, are widely used in various molding and extrusion applications. Such applications include films, sheeting, bottles, containers and the like. Aliphatic polyesters generally have low melting points and are therefore much less useful in typical industrial applications. However, it was recently reported that certain aliphatic polyesters based on monomers approved for food applications are useful as a chewing gum base. It would therefore be beneficial to provide for improved polyesters for chewing gum bases and for non-food applications and improved methods of making such materials.
The preparation of polyesters is described in a recent book by George Odian, Principles of Polymerization, 2nd Edition, pages 102-105 (1981), John Wiley and Sons, NY. Branched or crosslinked polymers can be prepared by using at least some monomers having a functionality of at least three. An apparatus for making such branched polymers is shown on page 132 of this same reference.
Conventional stirred and heated tanks are generally desired because the equipment is commercially available and can provide economy of scale. Unfortunately, these reactors are not easily adapted for stirring very viscous fluids. Moreover, once gelation occurs, it is nearly impossible to discharge the gel from the reactor. During the process, as the crosslinking reactions ensue, the reaction mixture becomes very thick and eventually gels. As such, agitation is impaired and eventually stops. Because the gelled mass becomes non-homogeneous, heat transfer is severely impaired resulting in pocket charring. Furthermore, the uniformity of the gel is compromised. Due to the extremely high rate of increase in polydispersity and molecular weight near the gel point, control of the reaction is almost impossible in a conventional stirred tank reactor. Additionally, once gel formation starts, it is uncontrollable due to the exothermic nature of the reaction and the severe limitations on mixing at significantly high viscosity.
The reactive extrusion step used in conjunction with the stirred reactor above was believed to be suitable equipment for the crosslinking the polyester because of its capability to heat, agitate, and discharge extremely viscous materials. However, use of reactive extrusion to prepare crosslinked polyesters requires residence times much too long to be economical or suitable for large-scale manufacture. Such a process, requires the removal of water as it is formed in order to increase molecular weight. For example, approximately 5-15% by weight of the reaction mixture is yielded as water by the condensation polymerization reaction. To insure that such large amounts of water are removed, requires slower operation of the extruder, otherwise only a viscous liquid rather than a gel will exit the extruder. As such, extruder screws must be slowed down or the extruder lengthened. Additionally, the extrusion equipment needs to be designed and configured to allow for a significant range in viscosity of the reacting polymer mixture. The feed is a thin liquid (at the high reaction temperature present in the extruder) and the product is a thick gel. This added complexity and low throughput results in a correspondingly high cost to produce the product by the reactive extrusion process described herein.
WO 98/17123 (1998) and WO 98/17124 (1998) PCT publications to Wrigley Jr. Company describe a gum base including at least one aliphatic polyester that is produced from glycerol, propylene glycol or 1,3-butylene glycol and an aliphatic dibasic acid containing 4 to 12 carbon atoms.
Nagata, M. et al, Macromolecules, Vol. 30, 6525 (1997) describe the synthesis and characterization of certain aliphatic polyesters based on pentaerythritol.
M. Nagata et al., Reactive Functional Polymers, Vol. 30, 165 (1996) describe the synthesis and enzymatic degradation of certain aliphatic polyesters based on glycerol and aliphatic dibasic acids containing 6 to 16 carbon atoms.
U.S. Pat. Nos. 4,319,017, 4,415,721 and 4,465,819 to Kosanovich et al disclose a process to produce thermotropic linear polyesters of aromatic dicarboxylic acids and diphenols. A two-step process is presented wherein a pre-polymer is prepared in the first step and subsequently reacted in a wiped-film reactor capable of high shear stress.
None of the references disclose a process for making a crosslinked branched polyester gel. Since the prior art processes are not adaptable to large scale manufacturing, premature gelation is much more likely to occur. Thus, it has been discovered herein a process that addresses the problem of premature gelation by providing a polyol-rich reaction mixture to make a pre-gel composition having hydroxyl end groups. Similarly, a carboxylic acid-rich reaction mixture is provided to form a pre-gel composition having carboxyl end groups. The pre-gel composition having carboxyl end groups and the pre-gel composition having hydroxyl end groups are mixed and processed in an extruder, continuous processor, or container to convert the mixture into a crosslinked polyester gel. Unlike the prior art process, the pre-gel compositions formed from the process individually do not form a gel even after days of being heated in a conventional stirred tank reactor at elevated temperatures.
The present invention produces in separate reactors a pre-gel composition having carboxyl end groups and a pre-gel composition having hydroxyl end groups, wherein the pre-gel compositions are subsequently mixed, processed and converted into a crosslinked polyester gel.
Some advantages of the present process of the invention are that because neither the pre-gel with carboxyl end groups nor the pre-gel with hydroxyl end groups will gel by themselves, both pre-gels can be substantially reacted to completion. Only minimal mixing is required prior to processing the mixture to form a crosslinked polyester pre-gel. Therefore, the residence time and temperature of the extruder can be substantially reduced and throughput improved as compared to the prior art. Furthermore, the water extracted content of the crosslinked polyester in this process is less than about 5 weight % and preferably less than about 1 weight %.
The present invention provides a process for making a crosslinked branched polyester from at least two pre-gel compositions. More specifically, the process can be used to prepare crosslinked branched aliphatic polyesters, which are particularly useful in food applications such as chewing gum bases, or the process can be used to prepare aromatic or non-biodegradable polyesters, which are useful in non-food applications, including cosmetics, baking agents, customized emulsions, inks, pigments and the like.
In an embodiment, the invention provides a process for making a crosslinked branched polyester gel from at least two pre-gel compositions comprising:
a) reacting precursor repeat units comprising (1) at least one polyol having three or more hydroxyl groups or esters thereof, (2) at least one aliphatic or aromatic polyfunctional acid or ester thereof, or a mixture thereof; and (3) optionally at least one long chain aliphatic carboxylic acid or ester thereof, or aromatic monocarboxylic acid or ester thereof, or mixture thereof, to substantial completion in separate reactors to produce in a first reactor a first composition comprising a pre-gel having carboxyl end groups and to produce in a second reactor a second composition comprising a pre-gel having hydroxyl end groups,
b) combining the first and second compositions to form a mixture; and
c) processing the mixture in an extruder, a continuous processor, or a container to convert the mixture into a crosslinked polyester gel.
In another embodiment, the invention provides a process for making a crosslinked branched polyester gel from at least two pre-gel compositions comprising:
a) reacting precursor repeat units comprising (1) at least one polyol having two or more hydroxyl groups or esters thereof, (2) at least one aliphatic or aromatic polyfunctional acid or ester thereof, or a mixture thereof having at least three functional groups; and (3) optionally at least one long chain aliphatic carboxylic acid or ester thereof, or aromatic monocarboxylic acid or ester thereof, or mixture thereof, to substantial completion in separate reactors to produce in a first reactor a first composition comprising a pre-gel having carboxyl end groups and to produce in a second reactor a second composition comprising a pre-gel having hydroxyl end groups,
b) combining the first and second compositions to form a mixture; and
c) processing the mixture in an extruder, a continuous processor, or a container to convert the mixture into a crosslinked polyester gel.
In another embodiment, the invention provides a crosslinked branched aliphatic biodegradable gum base and chewing gum composition made by the process of the present invention.
Additional advantages of the invention will be set forth in part in the description, which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawing, which is incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.